Title and description updated. I will ask upstream for an estimation of what it takes to implement this (complex!) feature. I'm also rising priority one step up to imply that the Wikimedia Phabricator team takes it into account even if this is a project to be tackled upstream. Low and not anything higher to set expectation right; it is really a feature with a complex implementation. Also added to Project-Management to have it under another radar.

Quick note, that performance would be my biggest concern as an end-user.
When we were using Trello, it would frequently cause my (firefox) browser to "freeze" for 30-60 seconds at a time, whilst medium to large size boards were loading, and dragging cards around was usually sluggish-feeling. e.g. https://trello.com/b/k5N0ivoM/research-and-data
Versus my experience with Phabricator so far, which has been that medium size boards load quite rapidly, and dragging cards around feels quite responsive. e.g. (1,600+ cards) https://phabricator.wikimedia.org/tag/flow/